The aim of this study was to investigate how the biomass of extramatrical mycorrhizal mycelia (EMM) is influenced by the addition of different phosphorus (P), potassium (K) and PK fertilizers in peatland forests with variable P and K availability. Four fertilizers were used: apatite, biotite, Rauta-PK (TM) (apatite and ferrosulphate) and a test fertilizer (50% apatite and 50% recycled iron phosphate). Forest plots with four different types of nutrient balance (deficient P and deficient K, deficient P and sufficient K, sufficient P and deficient K, and sufficient P and sufficient K) were studied. The effects on EMM biomass and ectomycorrhizal (EM) biomass in roots were estimated by ergosterol and phospholipid fatty acid (PLFA) analysis using in-growth mesh bags. Nutrients and rare-earth elements in EM roots surrounding the mesh bags were quantified and used as indicators of nutrient transport by the EMM in the mesh bags. The biomass of EMM was enhanced by P, K and PK deficiency of the trees, and EM fungal biomass in the roots was increased by P deficiency. The test fertilizer enhanced EMM biomass in all the plots studied, whereas the other fertilizers did not have any significant effect. No significant interactions between the P and K availability of host and mycelial fertilizer response could be detected. Deficiency of P or K or both in needles did not affect the concentrations of rare-earth elements in the tree roots. Earlier results from laboratory experiments have shown reduced carbon allocation to EM fungi under K deficiency, but this was not the case in these mature forests. Instead, we observed increased EMM biomass in response to both P and K deficiency.